Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Experimental satellite to have extended mission

Published on by zooks777Leave a comment

The Earth Observing-1 (EO-1) satellite, launched by NASA in late 2000, carries two remote-sensing instruments that may become operational devices in the future, given a proven track record on EO-1 and, of course, sufficient funding.  One, the Advanced Land Imager (ALI) is a test bed for sensors earmarked for the follow-on to the current Landsat-7 Enhanced Thematic Mapper+ (ETM+).  As well as the existing ETM+ six bands, ALI covers three others close to existing bands.  Whether by design or good fortune, two of these help define the important VNIR broad absorption by ferric iron minerals, neglected in remote sensing since the early days of the Landsat Multispectral Scanner.  Like the ETM+, ALI also carries a panchromatic band that spans the visible range, and which is aimed at providing a means of sharpening detail in images.  On ALI, however, this band has an improved resolution of 10 metres as opposed to the current 15.

More innovatory is the Hyperion instrument, a hyperspectral device that spans the visible to short-wave infrared range with 242 bands that are 10 nanometre wide.  Hyperion is comparable with airborne hyperspectral devices, such as AVIRIS.  In the experiment it captures data swathes that 7.7 km wide, made up from 256 pixels with a resolution of 30 m.  After initial difficulties with allowing for atmospheric effects  on the data, newly calibrated Hyperion data closely mimic mineral spectra.

Early work on EO-1 data in many fields, including geology, has proved sufficiently promising that NASA has given the mission a year-long extension.  Although data are restricted to only a few target areas suggested by the investigators, the extension is good news.  It is a reassurance about continuity of the Landsat programme, and a tantalising indication that the ill-fated hyperspectral Lewis satellite may be resurrected.

Information from: http://eo1.gsfc.nasa.gov/

Handy guide to the significance of meteorites

Published on by zooks777Leave a comment

Although the press made a great fuss in 1999 about the supposed discovery of signs of life in a meteorite reckoned to have been blasted off Mars by a giant impact, meteorites in general are the only direct means of developing ideas about how the Earth and the rest of the planets formed.  The market in meteorites is beginning to resemble the London Metal Exchange in its frenzied bullishness, but being collectibles it is rare types that command the highest prices, rather than their significance.   An excellent review of current ideas among meteorite specialists appeared in the 6 July issue of Science (Alexander, C.M.O’D., Boss, A.P. and Carlson, R.W. 2001.  The early evolution of the Solar Syetem: a meteoritic perspective.  Science, v. 293, p. 64-68).

Since development of theoretical ideas about the generation of the elements in stellar processes, it has become almost a cliché to ponder about the ultimate dependence of every aspect of the natural world on supernovae and their “seeding” of the galaxy with the chemical mix that is so familiar.  Even the nuclear processes involved are easily grasped.  Not so the means whereby star stuff assembled into planetary systems and laid the potential for life, plate tectonics and virtually everything else.  Alexander and colleagues from the Carnegie Institute of Washington span the interactions between physical conditions around young and rapidly evolving stars, derived theoretically, and the kinds of compounds that they can generate.  Meteorite chemistry and mineralogy, which are very diverse, put flesh on the bones of these ideas.  The tangible properties of different meteorite classes, together with their radiometric ages, are analogous to fossils in piecing together both planetary evolution and the various kinds of environments in the early Solar System.

One conclusion in the review that surprised me concerns the oldest materials known to us – calcium-aluminium-rich inclusions found in some chondrites, such as the famous Allende meteorite that fell in Mexico.  The pale inclusions contain evidence for the former presence of short-lived isotopes, such as 26Al.  So short are their half-lives that the delay between their nucleosynthesis and the assembly of the pale inclusions can have been a few hundred thousand years at most.  There are two possibilities: either such isotopes were generated by energetic particles emitted by the growing early Sun, or they had their source in supernova events.  Theoretical work on local genesis has so far failed to match the relative abundance of all such short-lived isotopes, derived from the amounts of their decay products found in pale inclusions.  It seems highly likely that collapse of a pre-solar cloud of matter to form the nebula out of which Sun, planets and the parent bodies of meteorites emerged was set in motion by shock waves from a nearby supernova.  They would have taken the form of a high-speed interstellar “wind” of gas.  Observed differences in oxygen-isotope proportions in meteorites were once ascribed to heterogeneous mixing of this explosive introduction of exotic matter.  However, the oxygen heterogeneities do not show up in the isotopes of other elements.  That mismatch has led to ideas of chemical fractionation during Solar System evolution, akin to that so familiar from the different behaviours of “light” and “heavy” oxygen during evaporation of water and its uptake in skeletons of living things exposed to different climates.  Differences in oxygen isotopes now form a strand in assigning different meteorites to sources at different distances from the evolving Sun, and in deducing that some rare meteorites did indeed come from Mars.

Clearly behind the hype surrounding promotion of staffed and unstaffed missions to Mars and the increasingly shady world of the meteorite trade, exciting research is being done.

Ice and prebiotic chemistry

The problem with ice on Earth is that it will not support living chemistry.  The process of crystallization excludes impurities from its structure, so that reactions between organic compounds cannot go on.  Comets are mainly ice, and frozen water is a common occurrence in the infrared spectra of interstellar clouds, along with a host of complex CHON compounds (over 100 discovered to date).  How organic molecules form in cold molecular clouds is a difficult problem, or at least it was believed to be until recently. 

Researchers at the NASA Ames Research Center in California have probed the structure of solid water under all manner of physical conditions.  Below a temperature of 200 K (about that of liquid nitrogen)  the hexagonal symmetry of ice, familiar from snowflakes, changes to the simpler cubic form.  At yet cooler temperatures 10 to 125 K), ice has no crystalline structure.  Like flint, it is cryptocrystalline or amorphous.  Curiously, even only a few degrees above absolute zero it can flow like a viscous medium, in the manner of glass, when irradiated with ultraviolet radiation.  The breaking and reforming of hydrogen bonds, as in liquid water, but slower, creates the conditions for retaining impurities and their chemical combination.  This odd behaviour at precisely the temperatures of molecular clouds explains their richness in organic molecules.  Quite probably comets form by accretion of such interstellar icy material.  The experiments revealed that warming of amorphous ice above 125 K does not result in a complete transition to cubic ice, that would exclude impurities.  Instead, around two thirds retains its odd properties.  The discovery strongly hints that much of the basic work of producing precursors to life’s chemistry is not only feasible in interstellar space, but that they can be delivered to planets as they collide with comets giving a kick start to the origin of life.

Source:  Blake, D.F. and Jenniskens, P.  2001.  The ice of life.  Scientific American, August 2001, p. 36-41.

Dinosaur nose mystery resolved?

Published on by zooks777Leave a comment

Popular animations of dinosaurs in Jurassic Park and Walking with Dinosaurs are palaeontologically speaking “state of the art”.  That is, except for the beasts’ noses.  A close observer will have seen Tyrannosaurus and Triceratops with nostrils high on their snouts, and appealing brachiosaurs apparently breathing through the tops of their heads.  Such reconstructions rely on the position of the nasal passages where they enter the skull, and in dinosaurs such bony nostrils are large and complicated.  Traditionally, dinosaur reconstructors have gone for the rear of the cavity for the positions of the fleshy nostrils.

Despite their extinction at the end of the Cretaceous Period, dinosaurs have many living close relatives, such as birds, crocodiles and some primitive lizards.  All of them have fleshy nostrils situated at the front of the bony openings.  For that matter, so do mammals.  For several years Lawrence Witmer of the College of Osteopathic Medicine at Ohio University (Athens) has been pondering on this, even setting up the DinoNose project.  Not only did Witmer apply the principle of parsimony to this intriguing issue, but noted the marks left on skulls by the blood vessels that supply the muscles that enable land vertebrates to snuff the air in many interesting and useful ways.  Such marks appear on dinosaur skulls, towards the forward end of the nasal openings.  The outcome is a fundamental revision of  dinosaur physiognomy (Witmer, L.M. 2001.  Nostril position in dinosaurs and other vertebrates and its significance for nasal function.  Science, v. 293, p. 850-853).  The next logical step is to seek signs that carnivorous dinosaurs did indeed snarl.

Cambrian Explosion:  Shropshire hits the news

As if by magic, nearly all animal phyla suddenly appear in the fossil record around 545 Ma, at the base of the Cambrian period.  The most famous of these are trilobites, a group within the phylum Arthropoda, for enthusiasts of which the Cambrian of Shropshire has long been a happy hunting ground.  Temporary excavations into the Protolenus Limestone of Comley have revealed a somewhat diminutive, though nonetheless startling relative that helps resolve the long-running debate over the origins of animals (Siveter, D.J., Williams, M. and Wlaoszek, D.  2001.  A phosphatocopid crustacean with appendages from the Lower Cambrian.  Science, v. 293, p. 479-481).  Superbly preserved in calcium phosphate, the tiny beast reveals great detail of its body parts, peeping from between a two-valve, spherical carapace.  It is possibly an early ostracod, and certainly a crustacean.  That such an advanced arthropod occurs close to the base of the Cambrian lends support to the view that animal diversification into extant phylla, and some vanished ones too, may have gone on far back into the Neoproterozoic.  The other view is that this radiation was explosive, beginning only 10 Ma or so before the base of the Cambrian.

The “long-fuse” hypothesis for the emergence and diversification of the animals is also supported by differences in the molecular biology of distantly related modern animals.  Assuming that accumulation of genetic change is steady, and can be calibrated by the coexistence of such groups as far back as the Cambrian, the “molecular clock” for animals probably started between 700 to 1500 Ma ago.  The problem, of course, is that only animals with hard parts or which miraculously had soft tissue rendered preservable by mineralization can assist palaeobiologists resolve the issue.  That is unfortunate, as such fossils occur only after about 5 Ma before the start of the Cambrian, and the large ones are exclusively Cambrian or younger.  The “explosion” was the sudden appearance of skeletal material, using calcium compounds such as carbonates or complex phosphorus-bearing material.  Such is the fascination with the detail of phyllogeny, that the trigger for the explosive emergence of hard parts is often overlooked.

See also:  Fortey, R.  2001.  The Cambrian Explosion exploded?  Science, v. 293, p. 438-439.

True polar wander

Published on by zooks777Leave a comment

One of the powerful bits of evidence that support continental drift are the plots of magnetic pole positions determined from rocks of different ages exposed on a modern continent, relative to that of the present pole position. Comparing such plots from different continents sometimes reveals similarities in their shapes over long periods of time, so that the plots partly match when they are superimposed.  In such fits, other parts of the plots diverge considerably.  Such comparisons are best explained respectively by the former unity of the two modern continents and their movement together, and their separation to drift independently.  The plots are illusory, and are called apparent polar wander paths.  For most of the Phanerozoic Aeon such palaeomagnetic data tie in well with other evidence for the formation of composite continental masses, such as Pangaea, and plate movements since the Triassic.  That provides confirmation of the basic assumption in palaeomagnetic studies that the Earth’s magnetic poles remain close to those of its rotation, bar some circulation around the axis and magnetic reversals.  It is tempting to use the same assumption for earlier times, in the absence of  easy fitting of the margins of continental segments and the sea-floor magnetic stripes that are the key to plate tectonics since the early Mesozoic.  If magnetic poles did move well away from the poles of rotation at any time in the past, that would play havoc with continental reconstruction.  True polar wander is something that many tectonicians “Dinnae care to speak aboot”!  That is not surprising, for another reason.  Use of the term imply mean two things: a long-term shift in the Earth’s magnetic polarity relative to its axis of rotation (to me that warrants the adjective “true”, and ); a shift in the relative position of the whole crust and mantle relative to the core, whose dynamism determines the magnetic field.

A recent review (Irion, R. 2001.  Slip-sliding away.  New Scientist 18 August 2001, p. 34-37) concentrates on evidence for the second usage.  There is evidence that suggests a 20° shift of all continents over a period of 2 Ma, in the Cretaceous.  This is dwarfed by a suggestion of a 90° shift in 15 Ma that span the time of the Cambrian Explosion, so that a continent could have moved from the pole to the equator at a rate far faster then anything known from Mesozoic to Recent sea-floor spreading.  One explanation is destabilization of the Earth’s angular momentum by concentration of all crustal mass and the effect of a massive mantle plume beneath it at high latitudes.  That would distort the Earth’s shape.  A planet’s rotation is most stable when it its shape is fat around the equator.  The opposite, a prolate spheroid, is least stable, and a polar supercontinent could result in such instability, restoring steady state if the whole caboodle slipped to lower latitudes.  That is what is proposed to explain some odd palaeomagnetic pole positions newly and accurately gathered from early Cambrian rocks.  Such a notion takes on its own momentum, because of its association in time with the explosive diversification of animals with hard parts.

It is not a fundamentally new idea, for Alfred Wegener suggested that the mechanism for his hypothesis of continental drift was Pohlflucht (flight from the poles) of continental mass.

Sniff ethylene and become an oracle

Published on by zooks777Leave a comment

In a small temple on the south slopes of Mount Parnassus in Ancient Greece sits a somewhat befuddled lady, her inhibitions definitely down and she sometimes becomes delirious and thrashes around.  The great and the good seek her opinion on matters of state.  Perhaps for almost 2 millennia, successive pythia (pythia) of the Delphic Oracle had a steady passing trade.  Writers from the classic times of Greek and Rome leave little doubt that the pythia’s powers stemmed from three things: a fissure and a spring lying at the centre of what became revered as the Sanctuary of Apollo, and a vapour that emanated from her chamber.  The Oracle was seemingly a matter of geology and its mystique. 

Indeed there are two intersecting faults passing right beneath the Oracle.  Suitably encouraged, a team from Florida State and Weslyan Universities, and The University of Louisville, USA has been studying all aspects of the site since 1995 (de Boer, J.Z., Hale, J.R. and Chanton, J.  2001.  New evidence for the geological origins of the ancient Delphic oracle (Greece).  Geology, v. 29, p. 707-710).  Where others failed before them, they have discovered evidence for a spring and traces of hydrocarbon gas leaking from a bituminous limestone cut by the faults at depth.  One of the gases in the mixture is ethylene, once used as an anaesthetic, and known to cause just the symptoms in the pythia described in ancient accounts of her powers.

Seafaring Homo erectus?

The first Homo erectus fossils recorded by Eugene Dubois came from Java.  Dubois was not so good at recording the geological context of his finds, and most of the later Javan discoveries were by local farmers.  Consequently the dates of first arrival of the erects are a subject of continual debate, recent suggestions being that erects maintained a hold in Indonesia until as late as 20 thousand years ago.  The incompleteness of  records also led to few finds of artefacts, so much so that doubt has been cast on any significant H. erectus culture.  Later work throughout Indonesia did reveal something quite astonishing, however.  The erects crossed Wallace’s line to colonise one of the easternmost islands in the Indonesian arc, Flores, where undoubted stone artefacts occur in rich beds of fossil bones.

Alfred Russell Wallace noted that the flora and fauna of western Indonesia are to all intents the same as on continental Asia, whereas those of the islands east of Bali are very different.  This empirical division is now known to have arisen through the emergence of land bridges between the western islands and mainland Asia as sea level fell to expose shallow seafloor during Pleistocene glacial periods.  Wallace’s line coincides with straits that are very much deeper than could ever disappear during falling sea level.  Reaching islands such as Flores demands that H. erectus must have devised means of crossing wide stretches of open sea.  Fission-track dating of zircons gives ages for bone beds with tools that range from 840 to 700 thousand years (O’Sullivan, P.B. et al. 2001.  Archaeological implications of the geology and chronology of the Soa basin, Flores, Indonesia.  Geology, v. 29, p. 607-609).  Erects crossed at least two major seaways to reach Flores, predating the first seafaring modern humans, who reached Australia around 40 to 60 thousand years ago by an enormous span of time.

Unwholesome fare

Since Raymond Dart’s notoriously bloodcurdling views on the dietary habits of early hominids and “the mark of Cain” appeared in his 1950s essay “The Predatory Transition from Ape to Human”, palaeoanthropology has sometimes tried to brush under the carpet evidence for cannibalism among our ancestors.  Considering the many funerary traditions practised today, some of which involve dismemberment and defleshing of corpses, it is easy to pass off cut-marks on fossil bones as indicating last rites.  However, when evidence of cooking turns up (and the Anasazi people of 12th century Colorado left plenty of evidence for that, including making anthropic soup), the common notice in pub restaurants, “Children served”,  takes on grim undertones.

Tim White, co-director of the Laboratory for Human Evolutionary Studies at the University of California (Berkeley) is a palaeoanthropologist who commands attention.  It was he who discovered evidence for Anasazi cuisine, and has subsequently maintained an interest in assessing evidence for cannibalism.  It does go back a long way, to evidence for the first European’s (H. antecessor) gustatory relish of their fellows at the 800 ka site of Gran Dolina in northern Spain, and similar signs in Neanderthal sites spaced by hundreds of generations.  The questions of, “How often?”, and, “Under what circumstances?”, are difficult to answer.  However, it was a part of the cultures unearthed by excavation.

Source:  White, T.  2001.  Once were cannibals.  Scientific American, August 2001, p. 48-55.

Radar analysis of Turkish earthquake

Published on by zooks777Leave a comment

The destructive Kocaeli earthquake (magnitude 7.4) of August 17 1999 involved horizontal slip of up to 5 metres.  Although it is possible to measure strains precisely using GPS arrays, many stations are needed to fully grasp strain patterns.  Interferometric processing of before and after radar data (InSAR) presents an opportunity to examine seismic strains over very large areas.  Displacements associated with the Kocaeli earthquake on the North Anatolian Fault, recorded by InSAR, extended for up to 60 km either side of the fault (Mayer, L. and Lu, Z. 2001.  Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry.  Geology, v. 29, p. 495-498). 

The fault runs parallel to the look direction of SAR beams from the ERS-2 satellite in its ascending orbits.  This fortuitous geometry charted relative motions in a horizontal sense on either flank of the major strike-slip fault system, with a precision of about 3 cm.  Interesting in its own right, the recorded strain helps understand how and where the elastic strain energy released by earthquakes was stored.  The key to energy storage is the rebound pattern associated with strain release during earthquakes, to which the InSAR results are an approximation.  This pattern depends theoretically on the displacement along the fault itself, the shear modulus of the rock involved and the depth to which faulting extends.  In the case of Kocaeli, faulting penetrated to between 6 and 15 km below the surface.  Because elastic strain builds up around active faults, it may be possible to use InSAR monitoring as a means of predicting the risk of future failures on dangerous faults, like the North Anatolian Fault.  Earthquake records show that successive failure migrates westwards along the Fault, getting ever closer to Istanbul.

Methane as the early “greenhouse” gas

Published on by zooks777Leave a comment

Various indicators, such as the presence of detrital uranium oxide and iron sulphide grains in sediments older than about 2.3 Ga and the appearance of terrestrial sediments stained red by the presence of ferric (Fe-3) oxides thereafter, have long been used to suggest that atmospheric oxygen was a mere trace before that time.  Generation of oxygen through photosynthesis by simple organisms, principally blue-green bacteria, could have led to an oxygenated atmosphere when their productivity exceeded the tendency for oxygen to be consumed by reaction with reducing agents, such as abundant ferrous (Fe-2) iron in sea water, and by burial of carbon-rich dead organic matter.  That method is a central plank in the Gaia hypothesis.  However, geochemical considerations suggest another scenario for oxygenation (Catling, D.C., Zahnla, K.J. and McKay, C.P. 2001.  Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth.  Science, v. 293, p, 839-843).  Unless carbon burial exceeded the rate at which reductants supplied to the outer Earth (including exposure of buried carbonaceous sediments) by geological processes consumed oxygen, the atmosphere would remain low in oxygen.

Lacking in oxygen, the early atmosphere would have been able to support build-up of methane from biogenic processes – today methane is soon oxidized to carbon dioxide and water.  Carbon isotope evidence suggests that early life was dominated by methanogens, and such organisms alive today are genetically very primitive.  Consequently, methane is a good candidate for keeping average surface temperature above the freezing point of water at a time when the Sun’s output of energy was considerably lower than it is now.  All hydrogen-bearing compounds become dissociated high in the atmosphere, to release hydrogen atoms, and they readily escape the Earth’s gravitational pull.  Fortunately, this does not happen now because the only significant H-compound, water, cannot rise above the tropopause.  The decline in temperature upwards acts as a cold trap for water.  Were this boundary not in place, and it is largely due to the presence of ozone in the stratosphere which absorbs radiation to give higher-level warming, Earth would long ago have lost most of its water, as did Mars and Venus.  In the early atmosphere, methane would not have been “cold trapped”, and nor is it today.  So, during that period, hydrogen would steadily have leaked from the Earth.

The chemical outcome of such a simple process would have been a steady decline in the reducing capacity of the Earth as a whole, for hydrogen is a powerful reductant.  Because most of our planet’s hydrogen was locked in water from the time of its accretion, its escape must have resulted in a net gain of oxygen somewhere in the Earth system.  Increased methane productivity by methanogen bacteria during the Archaean and early Proterozoic would have enhanced this tendency for the whole Earth to become more oxidizing.  Catling et al. argue that the continental crust became more oxidized, so that any gases released from it by metamorphism would become less reducing.  That would have reduced the tendency for immediate consumption of oxygen produced by photosynthetic organisms, culminating in its eventual ability to exist in the atmosphere in balance with biological processes at around 2.3 Ga.

Zircons’ window on the Hadean

The oldest tangible rocks that are not completely changed by deep-crustal metamorphism are those of Isua in West Greenland.  Interleaved with gneisses that originated probably from calc-alkaline intrusions are rocks formed at the Earth’s surface around 3.8 Ga ago.  The general scene represented by this Akilia Association is in many respects familiar – the operation of plate tectonics, rapid generation of what was to become continental crust, abundant evidence for the action of liquid water and even the isotopic traces of living organisms.  That 750 Ma after the Earth’s accretion the last two were present is no surprise.  The oddity is that, despite decades of effort, there is still no sign of continents older than 4 Ga.  That crustal rocks which had undergone considerable evolution from their mantle source did exist in the missing half-billion years emerged from the discovery of detrital zircons as old as 4.4 Ga in much younger Australian sedimentary rocks.  Some of the rare, tiny grains show isotopic evidence that the magmas in which they formed had contact with liquid water at the surface.

As well as containing sufficient uranium to allow the dating of single grains by the U-Pb method, zircons also contain hafnium, which is chemically very similar to zirconium.  Measurable quantities of 176Hf add to common 177Hf by the decay of 176Lu, giving a potential dating technique.  However, zircon contains only minute traces of lutetium, so that its 176Hf/177Hf ratio remains that of the ultimate source of its host rock.  Relative to hafnium, lutetium is more likely to remain in the residue left by partial melting of the mantle, or so theory suggests (geochemists can only deduce this from various lines of indirect evidence).  Consequently, mantle that has sourced continental crust builds up 176Hf from the time such crust formed., whereas continental crust has significantly lower levels.  Studying hafnium isotopes in very old zircons is therefore a means of seeking periods when significant amounts of continental crust separated from the mantle.  Because such tiny amounts of the radiogenic hafnium are involved, an accurate decay constant for 176Lu is vital (Scherer, E., Münker, C. and Mezger, K. 2001.  Calibration of the lutetium-hafnium clock.  Science, v. 293, p. 683-687).  Zircons from the oldest rocks in Greenland, Canada, Australia and South Africa fall into two, complementary groups; those with slight enrichment in 176Hf and those with slight depletion.  Simple geochemical theory seems to indicate that indeed magmas similar to those that contributed to formation of the bulk of continental crust did form as early as 4.4 Ga ago.  However, zircons with younger Archaean ages show little sign of deviant hafnium, which suggests that a large proportion of the mantle was not involved in early sial formation.  Hadean continental material no doubt formed, but not much.  That is no surprise, for involvement of surface-derived water in mantle melting above zones where earlier lithosphere returns to the mantle, whatever their form, seems inevitable in a planet noted for its high water content.  That is the basic “recipe” for the formation of silica-rich magmas.

Two things stem from this work: the probable futility of seeking Hadean continents; the unlikelihood that the chemical heterogeneity of the mantle stemmed from Hadean continet formation on a massive scale.

See also:  Kramers, J. 2001.  The smile of the Cheshire Cat.  Science, v. 293, p. 619-620

Yet more complexity

Published on by zooks777Leave a comment

The view that all manner of processes connected with climate – volumes of land ice, ocean temperature and flow, aspects of atmospheric composition and its motion, and the expansion and contraction of biological communities – are locked into the cycles of changing solar input steadily evolves into something less mechanical, as new data flows in.  The first serious doubts about Milutin Milankovic’s theory of astronomical forcing of the world’s climate, since oxygen isotope fluctuations in sea-floor sediments began to reveal the periodicities predicted by him, stemmed from a very different kind of deposit.  Devil’s Hole near Las Vegas, a fissure being slowly filled by calcite flow stone that precipitates from groundwater, presented a detailed record of oxygen-isotope variations over the last 600 thousand years.  Though showing the same patterns as ocean cores available at the time, Devil’s Hole revealed changes in continental climate that differed from those in land-ice volume by thousands of years.  Ice-core time series of Antarctic air temperatures also show that warming began up to 9000 years before the last four terminations of glaciation.  As more proxies for climate are devised, the more complex global climate shifts appear to have been.

The latest measure stems from fluctuations in the structure of compounds produced by marine algae as a result of shifts in sea-surface temperature.  Applied to sea-floor sediments deposited off California, an area influenced today by the southward, cold California Current, they reveal regional warming of the sea that began 10 to 15 thousand years earlier than the last five deglaciations of the northern hemisphere (Herbert, T.D. and 8 others  2001.  Collapse of the California Current during glacial maxima linked to climate change on land.  Science, v. 293, p. 71-76).  In cores south of the modern cold current, no such large discrepancies emerged.  In fact they accompanied the maximum extents of land ice.  It seems that, like the Gulf Stream, the California Current is prone to shutting down, but as a result of changed Pacific wind patterns in response to the North American ice sheets rather than to thermohaline deep circulation.  Here is an explanation for the vexing record from Devil’s Hole – regional climate shifts that do not “knock” Milankovic.

There is no doubt that changes in ice volume on the northern continents are the main characteristic of environmental change going back more than 2 Ma.  However, the mechanistic view that lots of ice means a cold, dry world and a great deal less points to warmth and more moist conditions is dead in the water as a useful paradigm.  Yet all models of climate are little more than Heath Robinson tangles of such reductionism, despite claims for their increasing incorporation of ideas that stem from measured realities.  As always, the devil lies in the detail, and Herbert et al.’s paper also shows from pollen records in the marine cores that dense warm-climate forests cloaked the Pacific seaboard during the last 5 glacial maxima.  For a vast area of western North America to be warm while ice sheets elsewhere were at their maximum should be a warning of unpredictable future climate shifts.

Growing concern about unpredictable and contrary change was amply expressed by a meeting of 1800 climate specialists in Amsterdam in early July.  They endorsed the distinct possibility of sudden shifts in regional climates that may stem from increased global warming, such as return of vegetation to the Sahara, aridity in the Amazon basin, and Europe’s plunging into a frigid climate as the Gulf Stream slows because of reduced thermohaline circulation (Pearce, F.  2001.  Violent future.  New Scientist, 13 July 2001, p. 4-5).

Magnetic stratigraphy works in the Devonian

Published on by zooks777Leave a comment

Using alternations of magnetic field intensity, and the patterns that they show over time, has been a standard method in stratigraphy for times back to about 200 Ma ago.  There is no sea floor older than that, and although reversals are known widely from earlier times, there is no continuity that allows its use.  Moreover, reversals are too widely spaced in time to allow for more than calibrating stratigraphic sequences.  Much finer stratigraphic resolution comes from direct and rapid measurement of the intensity of magnetization that can be induced in sediments from their content of various magnetic minerals.  The Ocean Drilling Programme and studies of loess sections in China have long established such magnetic susceptibility logging as a correlative tool.  Empirically, it works, and the loess studies suggested that variations relate to changes in global climate.  Its usefulness in marine sediments is now seen to relate to the production of massive amounts of very fine-grained magnetic minerals in tropical soil formation during warm-humid episodes.  Being so fine, the particles reach the most distant ocean basins after soil erosion.  Susceptibility seems to vary with global changes in the amount of continental erosion.

Detailed correlation between widely separated marine stratigraphic sequences of all ages is notoriously difficult.  Consequently, rapid methods based on magnetic susceptibility, which can produce near-continuous logs, have useful potential.  A team of Us, Spanish and Moroccan geologists has demonstrated its use in definitive correlation between Lower Devonian rocks found in Spain, Morocco and Bolivia (Ellwood, B.B. et al.  2001.  Global correlation using magnestic susceptibility data from Lower Devonian rocks.  Geology, v.  29, p. 583-586).

Climate and heavy breathing

Published on by zooks777Leave a comment

The kingdom of the eukaryotes rests on a very simple environmental economy.  Plants are producers of carbohydrate through photosynthesis, thereby generating excess oxygen from the photo- and molecular chemistry involved.  Animal consumers use up oxygen in their metabolism and return carbon dioxide, the ultimate source of carbohydrate, to the air.  A simple view is that animals contribute to global warming, whereas plants help cool the world.  Perhaps because of that “common sense” view, most environmental scientists take a very different line, linking it with volcanic exhalation of CO2, “capture of carbon through rock weathering and the burial of dead organic matter  in the global carbon cycle.  Greg Retallack of the University of Oregon is about to publish a reappraisal of the animal versus plant part of the C-cycle (in press, Journal of Geology) that is based on observed imbalances between the two opposed kinds of respiration.  Specialists in the C-cycle hold that there is a an overall balance, taking all components into account, whose inevitable result is the build up of oxygen in the atmosphere of an inhabited world.  Yet oxygen is extremely reactive and should quickly combine in mineral oxides and hydroxides – after all, the iron in an untended car reverts to its oxide ore in the space of a few decades at most.

Partly following James Lovelock’s Gaia hypothesis, Retallack focuses on the major fluctuations in atmospheric chemistry evidenced in the geochemical record, the most immediate being the see-saw fluctuation of modern levels of CO2 in the atmosphere – a 2% annual variation controlled by the waxing and waning of vegetation in the northern hemisphere (where plant cover is greatest) according to season.  One of the largest shifts in atmospheric CO2 concentration followed the evolution of land plants from about 450 Ma ago.  To thrive, they had to develop hard cellular material (lignin) that formed stems and trunks, which animals of the Palaeozoic were unable to oxidise efficiently.  Both living biomass and burial of undigested lignin drew down CO2 and boosted oxygen levels.  Animal evolution eventually exploited this “free lunch” through the humble termite and reptilian and then mammalian megafauns.  Retallack believes that heavy breathing that resulted from lignin digestion reversed the declining CO2 trend for the 200 Ma following the Carboniferous to Permian glacial epoch in Gondwana.  Though displaying some ups and downs, the Mesozoic saw a “greenhouse” world.  Removal of the mighty and extremely abundant herbivorous dinosaurs by the K-T mass extinction provided and opportunity for plant diversification.  Many Mesozoic plants evolved armour against browsing dinosaurs, exemplified by the surviving Andean “monkey puzzle” tree Araucaria.  Their demise removed the need, and the plant Kingdom’s evolutionary response was the appearance of grasses.  Reatallack points out that grass itself is not as good as lignin-rich plants in holding CO2, but grasslands encourage the development of thick carbon-rich soils that hold more than the soils of the forest floor.  It is this development that Retallack believes lay at the base of the decline in average global temperature through the Cainozoic, to culminate in the present Ice Age.  Unsurprisingly, proponents of the complexity and diversity of the C-cycle, particularly in the oceans, are disinclined to have truck with the hypothesis.

Source:  Pearce, F.  The Kingdoms of Gaia.  New Scientist, 16 June 2001, p. 30-33.

Carbonates and biofilms

Above the low level that is essential for their role in molecular “information” transfer, calcium ions pose a fatal threat to cell processes.  That is simply because excess calcium combines with carbonate ions to form minute calcium carbonate crystals within the cell when the solubility product of calcite is exceeded.  The solubility product is the concentration of calcium ions multiplied by that of carbonate ions, so that increase in one or the other can lead to supersaturation of calcium carbonate and imminent precipitation.  Because CO2 is an essential need for photosynthesis and a product of animal metabolism, this risk is always present.  In the most common photosynthesising bacteria, the cyanobacteria that have been around for at least 3.6 billion years, the drawing in of CO2 in the form of carbonate (CO32-) or bicarbonate (HCO3–) ions in water can result in supersaturation immediately around the cell.  When it occurs, the “blue-green” bacterial biofilms induce precipitation of calcium carbonate.  That is why such micro-organisms can act as reef builders, as they did to great effect during the early Precambrian (stromatolites), and also from Cambrian to Cretaceous times.

Calcite mineralization by biofilms is, however, a complicated process.  It is connected with highly reactive substances that cyanobacteria exude outside their cell walls.  Depending on their degree of ordering and the supply of calcium ions, these substances control the manner in which calcium carbonate precipitates.  The detailed biochemistry and the form of calcite biofilms obtained by study of modern cyanobacteria in different watery environments has allowed Gernot Arp and co-workers at the University of Göttingen to evaluate varying calcium and CO2 concentrations in ocean water since 540 Ma, and suggest differences in Precambrian oceans (Arp, G. et al. 2001.  Photosynthesis-induced biofilm calcification and calcium concentrations in Phanerozoic oceans.  Science, v. 292, p. 1701-1704).

Their studies suggest that up to the Cretaceous, the Phanerozoic oceans must have had higher calcium contents than they do today.  Microbial reefs formed in that period preserve details of the “blue-green’s” cell structure, suggesting that calcite was nucleated directly by the extracellular substances.   Vast burial of the calcite shells of planktonic metazoan organisms to form the Chalk deposits of Cretaceous age reduced very high levels to give the calcium-depleted oceans that prevailed during the Cainozoic.  Microbial carbonates of these younger ages show no structure.  The stromatolites that are so characteristic of Precambrian limestones are stuctureless too, although they show evidence of progressive build-up from myriads of thin layers.  Irrespective of the Precambrian oceans’ calcium content, this lack of structure can be explained by more dissolved CO2 that resulted from its higher concentration in the atmosphere.  About 700-750 Ma ago, stromatolites that contain calcified cyanobacterial cells appear, and that may signify the massive drawdown of CO2 from the atmosphere that is implicated in creating icehouse conditions on a global scale during the late Proterozoic Aeon.